Syrian President Bashar al-Assad could be tried for war crimes, says former Middle East peace envoy George Mitchell. Photograph: Sana/EPA

The former US Middle East peace envoy, George Mitchell, has said that the Syrian president, Bashir al-Assad, could be tried as an alleged war criminal over the brutal crackdown on opponents of his rule.

Mitchell, who was the US special envoy for Middle East peace until last May, said Assad could be tried for war crimes in the same way as Charles Taylor, the former president of Liberia who was this week found to have "aided and abetted" war crimes by a UN-backed tribunal in The Hague.

Speaking at an international security conference in Dublin, Mitchell was asked if he could envisage Assad facing a special war crimes court as Taylor did.

"Certainly, I don't think that anyone could rule that out at this time," he said.

The retired US senator, who also oversaw the peace talks that led to the Good Friday Agreement in Northern Ireland, called on Assad to step down and "permit a free, open choice of leadership".

He added that the international community should consider further sanctions against the Assad family and leading figures in his regime.

"I think there are more actions that could be directed at the regime and all those that are supporting what is occuring there particularly the grievous number of deaths and injuries at present."

Mitchell said he believed the Syrian people would eventually be able to "freely choose their own leaders" and that there would be a "democratic transition".

He denied that Europe and the US had abandoned the Syrian people to its fate.

The peace negotiator's comments came as a Syrian government newspaper accused the UN secretary general Ban Ki-moon of encouraging rebel attacks by focusing his criticism of violence on the government.

The editorial in the state-run Tishrin paper on Saturday comes a day after Ban said Assad's continued crackdown on protests has reached an "intolerable stage".

Tishrin said Ban has avoided talks about rebel violence in favour of "outrageous" attacks on the Syrian government.